Generally, glass materials are hard and also can be used in the form of films that coat substrates. However, when employing a melting method, a high temperature treatment is necessary to obtain a vitreous film. This limits the materials that the substrates and coating films can contain.
The sol-gel process is a process of obtaining an oxide in a solid state by: using a solution of an organic or inorganic compound of metal as a starting material; rendering the solution into a sol in which fine particles of metal oxides or hydroxides have dissolved through the hydrolysis reaction and polycondensation reaction of the compound contained in the solution; further gelling and solidifying the sol; and heating this gel.
The sol-gel process allows vitreous films to be produced at lower temperatures. A processes of forming silica-based films by the sol-gel process is disclosed in JP11(1999)-269657A, for example.
Generally, the silica-based films formed by the sol-gel process have lower mechanical strength than that of vitreous films obtained by the melting method.
JP11(1999)-269657A discloses a process for producing a silica-based film by applying an alcohol solution that is used as a coating solution to a substrate. The alcohol solution contains 0.010 to 3 wt %, in terms of silica, of at least one selected from silicon alkoxide and hydrolysate thereof (including partial hydrolysate), 0.0010 to 1.0N of acid, and 0 to 10 wt % of water.
The silica-based film obtained by this process has strength to an extent that allows the film to withstand the dry abrasion test. It probably cannot be said that the silica-based film has sufficiently high strength, but it has high mechanical strength for a film obtained by the sol-gel process. However, in the case of the silica-based film that can be formed by the process disclosed in JP11(1999)-269657A, the thickness thereof is limited to 250 nm maximum when consideration is given to obtaining an appearance that is good enough for practical use. The thickness of the silica-based film that is formed by the sol-gel process is generally around 100 to 200 nm.
It is possible to thicken the silica-based film by applying the coating solution multiple times to form a multiple layer. However, this may deteriorate the adhesion between the surfaces of each layer, reducing the abrasion resistance of the silica-based film. Another problem is that the process for producing the silica-based film becomes more complex.
Consequently, it is difficult to obtain a silica-based film that has a thickness exceeding 250 nm and has excellent mechanical strength.
A technique of forming an organic-inorganic composite film by the sol-gel process has been proposed. The film contains an inorganic material and an organic material that are composited. The sol-gel process is characterized by film formation that is carried out at a lower temperature. Accordingly, it allows a silica-based film containing an organic material to be formed. Organic-inorganic composite films that are formed by the sol-gel process are disclosed in JP3 (1991)-212451A, JP3 (1991)-56535A, and JP2002-338304A, for example.
In order to improve the mechanical strength of the silica-based film that is formed by the sol-gel process, it is desirable to heat-treat the silica-based film at 450° C. or higher. However, when an organic-inorganic composite film is heat-treated at a high temperature of this level, the organic material in the film will decompose. The restriction that the film must be heat-treated at a temperature in the range that does not cause the organic material to decompose limits the improvement in mechanical strength of the film to be formed, not only in the sol-gel process but also in the liquid-phase film formation methods other than that. Hence, it has been considered that it is difficult to form a silica-based film that has excellent mechanical strength when the film contains an organic material.